diatomaceous earth

You may recall diatoms from secondary schoolbiology. To wit, the term is a general one that refers to types of single-celled, microscopic and/or nearly microscopic species of algae. For millennia they have been living and floating and dying and drifting and reproducing, by the billion, every day. When a diatom dies, it leaves behind its hard silica shell. Over the millions of years, many of these trillions of diatom skeletons have formed into yet another layer of sediment in places beneath the sea. Of course, much more of the Earth was under the sea in the past than now, so we can find such deposits, called diatomaceous earth, on dry land. Here, it is mined and crushed and sold by the cartload.

Diatomaceous earth is useful for:

filtration - Diatomaceous earth is very fine, and packed into a filtration tank (along with layers of other substances of varying granularity) acts as an excellent water filter. It is used mostly in pools (rarely, these days) and fish tanks.

pest (bug) control - Gardeners will often spread diatomaceous earth throughout their beds (the plant beds, silly!). If you look at diatomaceous earth through a microscope, you will see many different unusual crystalline structures, but what you will see most is sharp edges. To an insect, this is like shards of glass. The insects walk through the fine powder and find that some of it sticks to them. This powder slices through their exoskeletons. The bugs' vital fluids seep out and the little beasties dehydrate and perish. *sniff* This form of pest control does not effect rodents, pets, children, or anything like that.